Blueglow n. 1. An atmospheric phenomena during both the twilight periods called dayspring and evenfall and occurring in two of the three stages of each of these twilight periods. Depending on the latitude of the observer, this event will last about twenty minutes, during the time when the sun is between 4° and 8° below the horizon.

The blueglow, also known as the blue hour is a time of twilight in dayspring or evenfall when during two (about twelve degrees total) of the three stages of the twilight period, an atmospheric phenomena that will usually last about twenty minutes, depending on the observer′s latitude. The blue color is most prominent when the sun is between 4° and 8° below the horizon.

This occurs once the Sun has dropped a significant depth below the horizon and when the afterglow takes on a predominantly blue shade. Too, if the sky is clear, the blueglow can be a colorful spectacle and indirect sunlight many tint the sky yellow, orange, red, and blue.

This atmospheric effect is caused by the diffusibility of the short blue wavelengths of visible light versus the long red wavelengths. During the blueglow, red light passes into space while blue light is scattered in the atmosphere and reaches the observer on the surface of the earth.

The period during even (twilight) in the morning and evening when the sun is significantly below the horizon. Blueglow (often called the blue hour) is that time when the light of the stars are starting to fade but the golden light of the sunglow has not yet shown itself. This residual, indirect sunlight takes on a predominantly blue hue, an effect caused by the relative diffusability of short blue wavelengths of light compared with the longer red wavelengths.

Normal blue light from the sky is a result of the scattering of sunlight, which results in a blue color perceived by the human eye. On a sunny day Rayleigh scattering gives the sky a blue gradient resulting in the blue being darker in the zenith but lighter near the horizon. Light that comes in from overhead encounters much less air mass (1/38th of the air mass) than encountered by the light coming along a horizon path. Therefore, fewer particles of air scatter the zenith sunbeam, and thus the light remains a darker blue.

However, at the dayspring and evenfall, the blue light takes on a different kind of pulchritude, one which makes it to appear to have a special blueglow.

The Blueglow (also known as the Blue Hour) appears when the sun is at a significant distance below the horizon, either at dayspring or evenfall when sunlight takes on a predominantly blue hue. During the blueglow, which typically last about thirty to forty minutes, red light passes straight into space while blue light is scattered in the atmosphere and thus reaches the surface of the earth.

Not having the sunglow is not necessary a bad thing because the blueglow can be just as remarkable of a display of light at this time of the day because of the high quality of light.

As the sun continues to drop to the horizon and the sunglow fades forthwith, I continue upward onto the mountain, and soon come to a road that is marked NF-103 on my GPS, so, I turn left onto the road and just past the NF entrance sign stating 16 days camping limit, I pull over into the first place to camp. I get out, walk up to the forest edge and find a fire pit with a large stack of logs that I could use for a fire if I wanted one. Because it is hastily turning dark, I grab my camera and take photos of the jammer in the fading light, the quickly fading sunglow and then the bright blueglow at the end of this day.